ATM RE: Pitch and Tile Tools

[Roger Ceragioli <rogerc@dakotacom.net>]

Hi, Jerry!

The way we do it at work is to cast individual squares of pitch, attach
these to a grinding tool with heat (a propane torch lightly touching the
back of each square until the pitch softens), and then attach each tile to
a pitch square (you can heat your tiles somewhat on a hot plate for this).  

Then put a layer of paper over your mirror blank to act as a release agent
(so the pitch doesn't stick to the mirror), and put your tile tool on top
of the mirror + paper layer to press out.  

Alternatively, you could cast a solid lap, let it cool, and then attach the
pitch squares via heat to the pitch layer.  After that rewarm the tool and
press out as above.

It doesn't matter if the tile sinks down quite far into the pitch, so long
as you get good contact with your mirror.  In fact, it's helpful if the
tiles sink into the pitch quite far, even so far that you partly grind with
the pitch.  The pitch will capture the tiles well, preventing them from
breaking off.  Just make sure that you have some light channels between the
tiles (cut them with a razor blade under cold running water) to allow the
abrassives to move around under the tool.  Grinding with pitch against
glass does absolutely no harm, and sometimes opticians purposely
"pre-polish" with aluminum oxide and pitch by itself.  This is known as
"grolishing".

Between grits, scrub moderately hard with a plastic brush to get the old
grit out.  Then if you're worried about contamination (usually not a
problem), you can lightly flame the exposed pitch with a propane torch.
This slightly melts the top of the pitch and permanently captures any extra
grit, trapping it in the pitch where it can't hurt anything.

Once you get a good fit of tool to mirror, grind away!  If you stop and
don't do any more grinding for a week, then before resuming, warm the pitch
and press it out just as you'd do for a pitch lap.  This kind of lap works
extremely well, is easy to make, and guarantees excellent contact between
lap and mirror.  You won't have to spend a long time grinding in the tiles
in the first place.

Hope this helps!

Roger Ceragioli